Three-dimensional radiative transfer modeling of the scattering polarization in the solar Hα line

The Hα line is one of the most important diagnostics of the solar chromosphere, but interpreting its scattering polarization remains challenging because of the combined action of magnetic fields, macroscopic motions, and the strongly inhomogeneous 3D structure of the atmosphere. We investigate the formation of the intensity and linear polarization of the solar Hα line in a 3D magnetohydrodynamic model of the solar atmosphere and assess the diagnostic information encoded in the observed scattering polarization signals. We solved the full 3D non-local thermodynamic equilibrium (non-LTE) polarized radiative transfer problem for a multilevel hydrogen atom using the PORTA code in a simulation that includes nonequilibrium hydrogen ionization. The calculations account for atomic polarization, Hanle-effect magnetic depolarization, horizontal radiative transfer, and macroscopic plasma velocities. The synthetic Stokes profiles were degraded to the spatial, spectral, and polarimetric resolution of available ZIMPOL observations to enable a direct comparison with measurements. The calculated center-to-limb variation of the Hα scattering polarization reproduces the observed overall trend and yields polarization amplitudes comparable to the observations. The simulations reveal a large spatial variability of the polarization signals arising from symmetry-breaking effects caused by horizontal inhomogeneities and atmospheric dynamics. The synthetic profiles naturally exhibit a variety of line-core asymmetries. By classifying the emergent Q/I profiles according to their degree of asymmetry, we find that their morphology is statistically correlated with the magnetic-field strength in the Hα formation region and is consistent with Hanle-effect depolarization. Multi-peaked profiles preferentially arise in regions with stronger magnetic fields, whereas asymmetric single-peaked profiles are associated with weaker magnetic fields exhibiting comparatively smaller vertical variations in strength. The scattering polarization of the Hα line is governed by the complex interplay between magnetic fields, velocity gradients, and 3D radiative transfer effects. Our results demonstrate the importance of accounting for 3D radiative transfer effects when interpreting Hα spectropolarimetric observations and confirm the diagnostic potential of this line for probing the magnetism and dynamics of the solar chromosphere.